Abstract

Abstract Specialized mechanoreceptors in the skin respond to mechanical deformation and provide the primary input to the tactile sensory system. Although the morphology of these receptors has been documented, there is still considerable uncertainty as to the relation between cutaneous receptor morphology and the associated physiological responses to stimulation. Labelled‐line models of somatosensory processes in which specific mechanoreceptors are associated with particular sensory qualities fail to account for the evidence showing that all types of tactile afferent units respond to a varying extent to most types of natural stimuli. Neurophysiological and psychophysical experiments have provided the framework for determining the relation between peripheral afferent or cortical activity and tactile perception. Neural codes derived from these afferent signals are evaluated in terms of their capacity to predict human perceptual performance. One particular challenge in developing models of the tactile sensory system is the dual use of sensory signals from the skin. In addition to their perceptual function they serve as inputs to the sensorimotor control system involved in manipulation. Perceptions generated through active touch differ from those resulting from passive stimulation of the skin because they are the product of self‐generated exploratory processes. Recent research in this area has highlighted the importance of shear forces in these exploratory movements and has shown that fingertip skin is particularly sensitive to shear generated during both object manipulation and tactile exploration. This article is categorized under: Models of Systems Properties and Processes > Organismal Models Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models Physiology > Physiology of Model Organisms

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(Upper) Images of the contact area on the finger at the specified forces. (Reprinted with permission from Ref . Copyright 2008 ASME) (Middle) Relation between contact force and contact area on the fingerpad averaged across ten participants. (Lower) Mean contact forces when people are asked to make judgments about surface friction, the presence of an asperity in a smooth surface, the roughness of raised‐dot surfaces and the temperature of different materials. (Reprinted with permission from Ref . Copyright 2006 Oxford University Press)

(a) Surface microstructure (profilometry) of four textures. (b) Spatial pattern of activation (spatial event plot, [SEP]) averaged over all SA1 afferents. (c) Average spike rates of SA1 afferents for 12 of the 55 textures that spanned the range from finest to roughest. (d) SD of the power spectra of the SEP derived from the SA1afferent responses, a measure of spatial patterning. (e) Mean correlations between SA1 spatial patterning (SEPs) and surface microstructure. (Reprinted with permission from Ref . Copyright 2013 National Academy of Sciences, USA.)

The axon innervating a Merkel cell gives rise to a several terminal branches that supply endings to tightly circumscribed (30–70 µm) clusters of Merkel cells. In other cases, the nodes of axons gave rise to en passant branches that formed extended oriented terminal chains contacting many Merkel cells extending over territories from 300 to 500 µm. The ratio of clumps to chains was approximately 1:4.